The Variscan Deformation Front (VDF) in Northwest Germany and Its Relation to a Network of Geological Features Including the Ore-Rich Harz Mountains and the European Alpine Belt

Abstract

The crustal basement of Northwest Germany can be interpreted as an “Avalonian Terrane Assemblage” subdivided by a roughly NW-SE (Hercynian) and SW-NE (Rhenish) running horst and graben system. In Late Devonian and Early Carboniferous times, this assemblage was flooded by the sea and mainly marine carbonates were deposited on the horsts and Stillwater shales in the grabens, as interpretable through magnetotelluric measurements. Dur ing the Late Carboniferous Variscan Orogeny, this terrain became the coal-rich foreland of the colliding Rhenohercynian belt. The shale-filled grabens reacted through folding and thrusting with different anticlinal patterns, the main carbonate covered horst in a still unknown way. This horst was the location of the Late Carboniferous basin center and of the inverted oil-rich Mesozoic Lower Saxony Basin (southwestern sector), respectively, with the so-called Bramsche Massif therein. It probably acted as an indenter for the evolution of the Variscan ore-rich Harz Mountains and forced the approaching Rhenohercynian orogen to stack the appropriate tectonic nappes by horizontal shortening to very high altitudes and the root into large depths. Based on seismic evidence this root is still an uncompleted crust/mantle transition zone with a deep reflection seismic and petrological Moho and a shallower hardly reflecting refraction seismic velocity Moho. The alternative, partly unsolved location of the Variscan Deformation Front in Northwest Germany may represent the new findings. The results may be supported by a comparison with features of the northern Alpine deformation belt.